Second-generation superconducting tapes, such as those based on YBa2Cu3O7-δ (YBCO) films, are being developed to carry large amounts of electrical current without electrical resistance. Such second-generation, high temperature superconductors (HTS) typically comprise biaxially textured (narrow out-of-plane and in-plane grain orientation distributions) buffer layers deposited on a metal substrate, such as a metal tape. It is known that the biaxially textured buffer layer enables high current densities in YBCO films, among others.
Several attempts have been made to grow sharply-textured YBCO films with high critical current density on flexible metal tapes. In one approach, a biaxially textured buffer layer was deposited using ion beam assisted deposition (IBAD) on a Ni-based alloy tape such as Hastelloy® (S. R. Foltyn et al., IEEE Transactions on Applied Superconductivity 9 (1999) pp. 1519–1522). The IBAD of a buffer layer of yttria-stabilized zirconia (YSZ) was the first demonstrated process to achieve biaxially textured buffer layers, and has led to the fabrication of several of the longest and best performing YBCO superconductors. It is generally accepted that texture development in IBAD YSZ is based on a growth competition mechanism. As a result, one disadvantage of this method is that thick layers must be grown in order to achieve good in-plane texture. For example, typically, buffer layers that are more than about 1,000 nm in thickness must be grown in order to achieve in-plane textures of less than 15 degrees full-width-at-half-maximum (FWHM). This problem is further exacerbated by the very low deposition rate (about 0.1 nm per second) needed to grow high quality IBAD YSZ. The combination of thick films and low deposition rates necessitates long deposition times (usually hours) to grow a buffer layer with a thickness greater than about 1,000 nm. Therefore, this process may not be suitable for industrial applications.
It is also known that IBAD of MgO has been used to achieve very good biaxial-texture in about 10 nm thick films using a deposition rate of about 0.1 nm/s (J. R. Groves et al., Proc. 2001 Intl. Workshop on Superconductivity, Honolulu, Hi. (Jun. 24–27, 2001), p. 3). This IBAD process can be about 100 times faster than IBAD YSZ. However, one disadvantage of this method is that the IBAD MgO method requires at least three additional layers in the buffer structure. The first is an amorphous seed layer, the second is a thick homo-epitaxially grown MgO layer, and the third is yet another layer for better lattice matching with YBCO. By requiring three additional layers in the superconductor, additional time and effort are needed to process the buffer structure. Also, the biaxial texture of MgO is very sensitive to the roughness of the underlying substrate and other factors. Therefore, it may be difficult to achieve high yields when manufacturing IBAD MgO-based buffer layers.
Inclined substrate deposition (ISD) of MgO without the assistance of ion beam bombardment has been shown to achieve high deposition rates (K. Hasegawa et al., Proc. of 16th ICEC/ICMC, Amsterdam: Elservier Science (1997), p. 1413; and M. Bauer et al. IEEE Transactions on Applied Superconductivity 9 (1999) p. 1502). The high deposition rates of this process can minimize the time for coating long wires. However, the quality of film produced by this method is poor compared to the quality produced using the IBAD method, and the c-axis in the MgO buffer layers is tilted off surface normal in this method. This makes the Jc anisotropic, and the Jc decreases greatly along the tilt direction. Additionally, the films deposited by the ISD method tend to have a rough surface with a pattern similar to “roofing tiles.”
In an additional approach, ion beam nanotexturing (ITEX) of YSZ has been shown to produce biaxially textured YSZ in a matter of a few minutes (R. P. Reade et al., Applied Physics Letter, Vol. 80, No. 8 (2002) p. 1352). ITEX is similar to IBAD, except that in the ITEX method, an amorphous YSZ layer is first deposited, then, an oblique Ar+ ion beam at an angle of about 55 degrees is used to bombard the amorphous film with O2 in a chamber. The result is a crystalline texture in the top surface of the amorphous layer. The ITEX method is very rapid, but yields a very poor in-plane texture of about 45°. An in-plane texture of about 10° or less is necessary in order to achieve good properties in the YBCO when deposited on the buffer layer.
Thus, there is a need in the art for a novel and robust IBAD process capable of producing long lengths of biaxially textured buffer layers. Such processes should create an ideal substrate for a subsequent deposition of YBCO or other superconductor. Such processes should substantially reduce process times. Such processes should result in an in-plane texture in the top surface region of the buffer layer of about 10° or less. Such processes should result in grain alignment on a large scale. Such processes should be suitable for manufacturing kilometer lengths of HTS coated conductors on flexible metal substrates at price and performance levels needed for numerous applications.